As for a device for adjusting a light amount, a diaphragm (iris) device is generally used. For example, in a diaphragm device 100 illustrated in FIG. 32, a plurality of (six blades in the illustrated example) diaphragm blades 101 serving as an iris are arranged on the circumference, and a driving point 102 of each of the blades 101 is moved in the direction of arrows 104 about a fulcrum 103 so as to reduce an aperture area of a center hole 105 through which incident light is transmitted.
In the diaphragm device 100, by expanding or contracting the aperture area of the center hole 105, the amount of transmitted light is adjusted. However, even if the diaphragm has the center hole 105 of which the aperture area is minimized, a correct exposure may not be obtained at the time of photographing a bright outdoor image. Further, at a diaphragm position where the center hole 105 has a relatively small aperture area, resolving power may be lowered by diffraction of light, thereby deteriorating a photographic image quality.
Accordingly, as a device for adjusting an amount of transmitted light, for example, a light transmittance variable filter disclosed in Patent Literature 1 is constituted by a disc-shaped filter in which a density of a light reduction material is continuously changed along the circumference. The amount of transmitted light of the filter is controlled by adjusting the rotational position of the disc-shaped filter.
In a light transmittance variable filter disclosed in Patent Literature 2, electrodes are attached on both surfaces of a disc-shaped dielectric elastomer placed on an optical path, and a voltage to be applied between the electrodes is adjusted to control a film thickness of the disc, thereby controlling the amount of transmitted light of the filter.
When the light transmittance variable filter disclosed in Patent Literature 1 or Patent Literature 2 is used instead of the diaphragm device illustrated in FIG. 32, a correct exposure may be obtained at the time of photographing a bright outdoor image. Thus, reduction of resolving power, which is caused by light diffraction at a diaphragm position with a relatively small aperture area, may be avoided. However, in the light transmittance variable filter, it is impossible to perform photographing which defocuses a background by expanding or contracting the aperture area to adjust the depth of field, which is an advantage of a diaphragm device.
That is, a diaphragm device and a light transmittance variable filter are the same from the standpoint of a function of being capable of adjusting an amount of transmitted light. However, since they have both merits and demerits in other functions, it is difficult to say which one is better.
Accordingly, as disclosed in Patent Literatures 3 and 4, there has been suggested an image capturing apparatus employing both a diaphragm device and a light transmittance variable filter (variable ND filter) in combination. In such an image capturing apparatus, a correct exposure may be obtained in any photographing conditions. Thus, it is possible to avoid reduction of resolving power, which is caused by light diffraction at a diaphragm position with a relatively small aperture area, as well as to adjust a depth of field. However, by being equipped with two light amount adjusting means, a cost is increased, and thus, no application to a practical machine is realized.